Effect of Non-uniform Cross Flow on Flow Induced Vibration of Every Heat Exchanger Tube in Bundle

Abstract

Non-uniform cross flow goes across the heat exchanger tubes of steam generators in nuclear plants, resulting in serious vibrations and wears. The pitch velocity varies among tube bundles and each tube is subjected to different fluid forces, causing inconsistent vibration response of each tube. To get the flow induced vibration characteristics of the whole heat exchanger tube bundle, an experimental setup of threespan tube bundle with 28 circular tubes and 10 semicircular tubes was designed. The test section has a glass window at the top for vision measurement. Coded reference points were attached on the top of the heat exchanger tubes to improve the accuracy of vision measurement. Different codes correspond to different tubes, enabling the image processing program to automatically identify each tube. The center dots of the reference points also provide a stable tracking target for the image processing program. A baffle with 1/3 opening was set upstream to create nonuniform cross flow to the tube bundle. Vibration displacement of each tube was measured with a high-speed camera and vibration acceleration of a typical tube was measured by an accelerometer, while the tube bundle was subjected to this nonuniform cross flow. In order to contrast with nonuniform cross flow conditions, the vibration of the tube bundle facing a uniform orifice baffle was tested. The results show that the heat exchanger tube bundle is more prone to fluidelastic instability (FEI) under nonuniform cross flow conditions compared to uniform orifice baffle conditions. And the vibration amplitude and frequency of each tube of the whole bundle is unique under nonuniform cross flow conditions. Four local resonance regions coexist in the tube bundle when flow induced vibration instability happens. The most significant fluidelastic instability resonance happens to the central tubes at the bottom three columns facing the opening of baffle. The peak frequency of vibration in this region is consistent with the firstorder natural frequency of the tube with one fixed support in middle and one simple support on top. The non-uniform distribution of the tube bundle vibration response will lead to inconsistent wear of the tubes located in different regions, which adversely affects the maintenance and service life of steam generators. The FEI critical velocity under the nonuniform cross flow was compared with the results predicted by five empirical correlations. Connors correlation and Chen correlation are conservative. The FEI critical velocity predicted by Connors correlation and Chen correlation are 502% and 247% lower than the experimental value, respectively. To predict the FEI critical velocity more accurately under nonuniform cross flow conditions, the Gorman correlation, which is closest to the experimental value, is modified. The results provide guidances for the design of heat exchanger tube bundles of steam generators.

Keywords

• vibration of every tube in bundle
• non-uniform cross flow
• fluid-elastic instability
• visualization measurement